The typical epicyclic or planetary gear system basically has a sun gear provided with external teeth, a ring gear provided with internal teeth, and several planet gears located between the sun and ring gears and having external teeth which mesh with the teeth on the sun and ring gears. In addition to its gears, the typical system has a carrier to which the planet gears are coupled. Typically the sun gear, the ring gear, or the carrier is held fast, while power is delivered to and taken from the remaining two components, and thus power is transferred through the planetary system with a change in angular velocity and an inverse change torque. However, in some epicyclic systems all three rotate.
The sun and ring gears for all intents and purposes share the same axis—a central axis—while the planet gears revolve about radially offset axes that are parallel to the central axis—or at least they should be. Often the offset axes and the central axis are not parallel, and as a consequence the planet gears skew slightly between sun and ring gears. This causes excessive wear along the teeth of the planet, sun and ring gears, generates friction and heat, and renders the entire system overly noisy.
The problem certainly exists in straddle-designed planetary carriers. With this type of carrier the pins on which the planet gears rotate extend between two carrier flanges in which the pins are anchored at their ends. The carrier experiences torsional wind up which causes one carrier flange to rotate slightly ahead of the other flange and produce a poor mesh between the planet gears and the sun and ring gears. Each pin at its ends in cross section should possess enough shear area and section modulus to withstand the shear forces and bending moments exerted on the pin by the flanges.
Another type of epicyclic gear system utilizes a single flange carrier and flexible pins anchored in and projected from the flange. In this arrangement the single carrier flange is offset axially from planet gears, and the carrier pins project from that flange into—and indeed through—the planet gears. Each carrier pin has one end anchored in the carrier flange and at its other end is fitted within a sleeve which returns back over the pin, yet is spaced radially from the pin, to support the planet gear—a double cantilever so to speak. U.S. Pat. No. 3,303,713 to R. J. Hicks shows such a double cantilevered arrangement. But the sleeves occupy space which could otherwise be utilized to enlarge the pin diameter, and when an antifriction bearing is interposed between the sleeve and the planet gear that is around the sleeve, even less space is available for the pin.
Essentially, the double cantilever, with a sleeve interposed between a planet gear and the pin about which the gear rotates, reduces the cross section of the pin and of course the shear area and section modulus. This can reduce the torque capacity of the system, so frequently more pins are added to increase the available total cross-sectional area of the pins and regain some of the lost torque capacity. Using more pins necessarily spaces the peripheries of the planetary gears closer together. Sometimes the number of pins required to achieve the required torque capacity will result in interference between the planetary gears.